Foam stripes parallel to the coast.

On my way to Heligoland the other day I noticed a phenomenon that I found really intriguing and that I should probably be able to explain. I first saw it on the screen of the boat’s web cams when we were about to leave the port of Hamburg. Unfortunately I could, at that point, only take a photo of the screen (but see how I excited I was to actually take a photo of the screen? ;-)).

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Can you spot what I mean? Yes, that foam stripe running pretty much parallel to the pier! The place where it bends is right in front of our boat, which you see the railing off as that white stuff at the bottom of the screen).

But then, on Heligoland, I saw it again and became even more intrigued. Right in front of the place I stayed at, you could see it looking down the coast to your left…


…and to your right!


Here we can also see the stripe bending at some point, but here again the bend coincides with a change in the coast line. Similarly to what we saw in the port of Hamburg above, the stripe stays at more or less the same distance from the coast, so it is parallel while the coast is straight, and bends out when there are obstacles (like the catamaran above or the rocks below).


So how do we interpret the whole thing? I am not quite sure. I seem to have a very vague recollection that it should have something to do with half a wave length of the dominant wave, and foam collecting in a node point. And that makes sense intuitively. Except that I have several (ha! understatement of the month) minutes of video footage of the above, and I cannot for the life of me spot anything that would explain the stripe. If it is a node point, it is a very well-disguised one and I am surprised the foam can find it!

But there must be something different about that location than about all the other places closer to or further away from the coast. Any ideas, anyone?

5 thoughts on “Foam stripes parallel to the coast.

    1. Mirjam

      Oh, interesting point, thanks! It kinda does look like one of those Langmuir stripes, yes. But I don’t think that’s what it is. In both instances that I observed those stripes in, there was a) only one stripe, and b) they were parallel to the coast rather than aligned with the wind (you probably can’t tell from the pictures, but the wind was coming from the left, from the sea, at an angle of maybe 45 degrees to the coast. What I would expect to see in Langmuir circulation would be several rows, going pretty much parallel to the wind (like here:

      But maybe it still is Langmuir circulation? Both times the stripe formed very close to a sea wall (both sea walls probably reaching a meter or so above sea level), so maybe the wind just off the sea wall was deflected and going parallel to the sea wall? That would only be the case for a small distance from the wall, therefore maybe only causing this one stripe? What do you think?

      1. Pengcheng

        Interesting observation! My first guess is also Langmuir circulation, but only one strip… Could it be related to waves reflected by the sea wall? Then there will be a net mass transport both to and from the sea wall due to waves, which creates a convergence zone?

  1. Pingback: Foam stripes parallel to the coast, take 2 | Mirjam S. Glessmer

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